They change from processes involved with the rock cycle. See related question on the rock cycle.
Any kind of rock can change into another rock through the process of metamorphism. E.g. limestone can change into marble or shale can turn into slate.
Rocks change in the rock cycle by intense heat, pressure, or erosion. Rocks become either igneous or metamorphic through heat/pressure, and sedimentary rocks are formed when rocks erode and compress...
A combination of increased temperature, a decrease in confining pressure or addition of volatiles can all act to cause rock to melt.
Yes. Through the rock cycle.